1. Field of the Invention
The present invention concerns a method for producing a color filter for an image sensor suitable to formation of colored images of a highly fine color filter used in an image sensor (CCD, CMOS, etc).
2. Technical Background
As a method of producing a color filter for use in an image sensor, a dyeing method, a printing method, an electrodeposition method and a pigment dispersion method have been known. Among them, the pigment dispersion method is a method of using a colored radiation-ray sensitive composition in which a pigment is dispersed in various photosensitive compositions and producing color filters by a photolithographic process, and since the pigment is used, the obtained coating film is stable to light, heat or the like. Then, since patterning is conducted by the photolithographic process, the positional accuracy is high and it is used generally as a method suitable to production of a color filter for use in large screen and highly fine color display.
In the production of the color filter by the pigment dispersion method, a ray-sensitive composition containing a pigment is coated and dried on a glass substrate or the like by a spin coater or a roll coater, to form a coating film and the coating film is exposed patternwise and further developed. Then, a heat treatment (post-baking) process is applied for improving the solvent resistance, development resistance and heat resistance of the resultant pattern images to promote hardening of the coating film thereby obtaining permanent coating films of high durability. By way of the processes described above, the processes after the second color can be conducted stably as much as possible and a color filter can be obtained by repeating the operation for several times in accordance with a desired number of hues.
Then, in a case of producing a color filter for use in a liquid crystal display device, a light shielding layer referred to as a black matrix is usually disposed with an aim of improving the contrast for shielding light between each of color pixels. Accordingly, a pattern of a forward taper is usually formed as a pattern profile for each of the pixels in order that a gap is not formed between each of the pixels and the black matrix.
On the contrary, in a case of a color filter for use in an image sensor, since highly fineness and high density with the length for one side of a pixel being 5 μm or less is desired, it is necessary to make each of the pixels independent without providing the black matrix or the like. However, since each of the pixels is made independent, disadvantage often occurs upon forming each of the pixel to result in a problem that a pixel with a rectangular profile for the pattern cross section can not be obtained. The disadvantage includes occurrence of “tailing” in which pattern images are deformed in the direction of increasing the contact area with the substrate, or “thermal flow” in which corners of a pixel are rounded upon conducting the post-baking. The tailing or thermal flow is a minutes deformation of the pattern which gives no problem in existent large pixels but entails decrease of the film thickness or color mixing since the inter-pixel distance is shortened in a small pixel and each of the pixels can not be constituted with a desired color. Particularly, in a color filter for use in an image sensor requiring highly fineness, the pattern profile for each pixel is one of important factors.
Further, in a case of the pigment dispersion type described above, since the grain size of the pigment is large to scatter the Ultraviolet radiation upon exposure, the resolution power is not improved and, accordingly, this imposes a limit on coping with the requirement of higher fineness. Further, since the grain size of the pigment has a distribution, it is difficult to form a uniform pixel with no color shading and it has a limit for the application use of the image sensor that requires a fine pattern. As a technique for overcoming the problems described above, JP-A No. 6-75375 described an example of using a dye thereby improving the resolution power and making the color distribution in the pixel uniform.
For the method of using the dye, a dying method of forming pattern images on a substrate by using a photosensitive composition containing a dye and subsequently dyeing the thus formed pattern images. However, the dyeing method involves a problem that it is difficult to control the degree of dyeing and the processes are complicated compared with other methods. Accordingly, as means for overcoming the drawbacks of the dyeing method and the pigment dispersion method, a method of producing a color filter for an image sensor by using a photosensitive composition containing a dissolved dye has been studied.
Particularly, in an application use for image sensors, it has been required to decrease the thickens of a color filter in view of a commercial demand of decreasing the size of image sensors and, specifically, it is required that the thickness is 1.5 μm or less. In order to maintain a sufficient chromaticity in a color filter for such a reduced film thickness, it is necessary to incorporate a great amount of a dye in a hardenable composition that forms a color filter (at least by 10% by mass or more in solid content). However, in a case where the content of the dye is increased excessively, since the ratio of the curable component is decreased relatively, the photo-curability of the composition, heat resistance after photo-curing, developability in a non-cured portion and the film thickness retention of the photo-cured portion are lowered. Then, they involve significant problems of resulting lowering of the film thickness retention in the photo-cured portion during the development process to lower the chromaticity, and further pattern deformation entailing ‘tailing’ or ‘thermal flow’ during post-baking to change the chromaticity.
Particularly, the problem of the pattern deformation during the post-baking is conspicuous in the dye system with a larger content compared with the pigment system. The pigment system color filter by the pigment dispersion method described above has high heat resistance for the entire photo-cured film due to the dispersion of the heat stable pigment in the cured film and can provide a pattern of less losing the shape due to “filler effect” thereby less resulting in the problem of the pattern deformation described above.
As described above, since the highly fineness is required in the color filter for use in the image sensor, a dye-containing curable composition has been studied. Particularly, for satisfying also the requirement for reducing the film thickness together with higher fineness, it is necessary that the dye is incorporated at a high concentration in the curable composition that forms the color filter. However, “filler effect” as in the pigment system can not be expected in a case of the dye system. Accordingly, the dye present at high concentration in the cured film results in deterioration of the film property and, as a result, entails tailing or thermal flow to worsen the pattern profile of the pixel.
In connection with the situations described above, JP-A No. 2000-19728 describes a technique of containing a 1,3,5-triazine derivative and N-phenyl glycins in combination in a photo initiator component as a technique of obtaining pixels with good pattern profile. However, since the method is directed to the pigment system, this can not improve the worsening of the pattern profile which may possibly occur in a case of the dye system.
Further, JP-A No. 11-153708 describes applying Ultraviolet radiation irradiation (together with heating) to a film after development by using a pigment type resist. However, this is also directed to the pigment system and does not intend to ensure the resist shape as an object of post-baking but intend to prevent thermal decomposition of an underlying flattening film during the post-baking. Further, application of Ultraviolet radiation irradiation together with post-baking by using the pigment system composition has been also be known, for example, in JP-A No. 10-339959; This method is also directed to the pigment system in which Ultraviolet radiation irradiation is also conducted in addition to a drying treatment (post-baking) with an aim of preventing color contamination at or after the second color. Further, a technique of applying Ultraviolet radiation irradiation together with heating to a positive type resist film has also been known, for example, in JP-A No. 10-261571. In a case of the positive type resist, heating and UV light irradiation are further conducted to a non-irradiation region upon pattern formation, that is, to a not yet reacted region. None of them concerns the dye system. Accordingly, it does not intend to overcome the worsening of the pattern profile which comes into question when constituted as a dye system that requires a great amount of the dye as described above and discloses nothing about the pattern profile.
Further, a method for producing a solid image device by using a dye system photosensitive composition is also disclosed in JP No. 3309514. In this method, the photo-curing temperature is elevated to 60 to 200° C. and elevation to the temperature is conducted gradually thereby intending to promote curing while avoiding the heat deformation by rapid heating in a case of applying heating together with light for improving the photo-curing efficiency with a view point of merely preventing deformation by heating. However, in a case of the application use for the image sensor constituted with a fine pattern, even gradual temperature elevation within the temperature range described above is still insufficient to completely avoid deformation caused by the tailing or thermal flow of the pattern.
In addition to those described above, it has also been proposed to improve the heat resistance or the like by using a photosensitive polymer compound introduced with a photosensitive vinyl group by way of an isocyanate group as described in JP-A No. 6-230212, or improve the heat resistance of pattern after curing by using an acrylic resin with addition of an unsaturated carboxylic acid to a polymer comprising a monomer having an epoxy group and an unsaturated double bond as described in JP-A No. 9-106072. However, since the alkali soluble resin has no double bond in the former, the pattern profile has not yet been improved so as to be suitable to an image sensor requiring high fineness and high density. Further, since heat curing is conducted above 200° C. in the latter, discoloration of a dye may possibly occur in a case of using a dye sensible to heat.    [patent document 1] JP-A No. 6-75375    [patent document 2] JP-A No. 2000-19728    [patent document 3] JP-A No. 11-153708    [patent document 4] JP-A No. 10-339959    [patent document 5] JP-A No. 10-26157    [patent document 6] JP No. 3309514    [patent document 7] JP-A No. 6-230212    [patent document 8] JP-A No. 9-106072